1. Technical Field
The present invention relates to an ink composition, a recording method, a printed matter, an ink cartridge, and an inkjet recording device.
2. Background Art
Inkjet printers are widely used because they are relatively quiet and enjoy low running costs, and many printers capable of printing color images on plain paper are now widely available on market.
However, it is extremely difficult to satisfy all the requirements of images produced using such printers, such as good color reproducibility, abrasion resistance, durability, light resistance, drying property, feathering, color bleed, duplex printing, and ink discharging stability. As a result, the ink used is selected based on the particular application.
In general, ink used in inkjet recording is mainly composed of water, with a coloring agent and a hydrosoluble solvent such as glycerin to prevent clogging. As the coloring agent, dyes are widely used for their coloring and stability. However, the light resistance and water resistance of images produced using such dye-based ink are inferior. Water resistance can be improved to some degree by using specialized recording media having an ink absorbing layer, but is not satisfactory at all when it comes to plain paper.
To compensate for such defects, ink using a pigment has begun to be widely used as a coloring agent in recent years. Although pigment ink is successful and superior to dye ink with regard to light resistance, water resistance, etc., the coloring of the pigment ink is degraded by coherence of beams of light having different wavelengths and phases produced by multiple reflections of the beams of light within the pigment. For this reason, pigment ink is considered to be inferior to dye ink in general with regard to coloring.
In an attempt to compensate for degradation of coloring of such pigment ink, pigment particulates coated with a resin are used. According to this method, the resin further improves the fixing property and the gas resistance property of pigment ink and greatly contributes to the dispersion stability thereof. However, currently, pigment ink is not still on a par with dye ink with regard to gloss.
JP-2002-138233-A discloses a crystalline organic color pigment having a half value width of 0.4° or less at the maximum diffraction peak of a CuKα X-ray diffraction spectrum. Although this pigment is successful to some degree in improving the coloring thereof by reducing the half value width to improve the crystallinity of the pigment, that coloring is still unsatisfactory.
C.I. Pigment Yellow 155, which has a disazo skeleton represented by chemical formula 1 illustrated below, is widely used because of its excellent coloring and light resistance. However, being a pigment, coloring is still inferior to that of a dye.
One effort to improve coloring involves reducing the particle size of the pigment. Although successful in improving the transparency of the pigment, which contributes to improvement of gloss, the impact on coloring is still limited.

In addition, it is possible to make the coloring of a pigment closer to that of a dye by reducing the crystallinity of the pigment. However, if the pigment loses crystallinity and becomes non-crystal like a dye, pigment characteristics such as light resistance and gas resistance are drastically worsened. As a consequence, using such pigments is impractical.